Using a background image in UV editor? To trace tattoos with paint mode
All steps will be done in UV Image Editor (Paint Mode).


Create New Texture Mask, name it as you want.
Choose Mask Mapping > Stencil.
Go to Properties Panel > Texture.
Select Brush Mask - "Your texture name".
Load your tattoo image.

If you have it white on black color you can skip those steps.
Under Image Sampling Tab select Calculate.
Under Colors Tab check Ramp and switch color handles and adjust if needed.
Go back to UV Image Editor and trace your mask.



  Note: Manipulating Stencil in Mask mode is a bit different. 
  
  
  Alt+RightClick - Move
  Shift+Alt+RightClick - Scale
  Ctrl+Alt+RightClick - Rotate